The present invention relates to a method for determining, at a measuring point on an electricity transmission network, the distance between this point and a fault affecting a line in the network the resistance and inductance per unit length of which are known, or for determining the resistance of this fault, comprising deriving instantaneous measurement signals of the voltage and current on the line, deriving, from these measurement signals, instantaneous processing signals of voltage and current related to the respective measurement signals by transfer functions, and giving the said distance or the said resistance the value which, for at least one group of simultaneous processing signals of voltage and current conforms to Ohm's Law applied to the line considered as resistive and inductive.
Methods of this type are known and examples are given of them in French Pat. Nos. 1 376 746 and 2 057 232, and in U.S. Pat. Nos. 3,369,156, 3,612,989 and 3,593,124.
In the classic forms of these earlier techniques, Ohm's Law is applied to the faulty line which is assumed to be an open circuit and exclusively resistive and inductive; there results an equation incorporating the two unknowns which are on the one hand the resistance of the fault and on the other hand the distance separating this fault from the measuring point.
The value of this distance is obtained either by using Ohm's Law, applied to the resistances and the inductance of the line, at the moment when the current in the faulty line is zero, which suppresses the effect of the fault resistance, or by using Ohm's Law for two groups of voltage and current measurement signals corresponding to different measurement instants.
These known methods, which have given full satisfaction, suffer however from a limitation which, in the present context of continuous increase in the length of lines, is becoming more and more inconvenient and which involves the fact that these methods take no account of the distributed capacitances of the electric lines.
However, if the distributed capacitances of the lines have been neglected up to now, it is because no way could be envisaged to take account of them without leading to a substantially increased complexity in the methods to be put into effect.
In this context, the object of the present invention is to provide a method which, without substantially increasing the complexity of previously mentioned known methods of distance measurement, enables the distributed capacitances of the line to be taken into account in the methods of direct measurement of the distance of a fault.